Heated catalytic bead pellistor-type devices have been used as sensor elements in various types of detectors of combustible gases. Such devices operate by measuring heat generated by combustion of the gases. Some gases can combust rapidly within the outer regions of the devices. Others combust more uniformly within the device. In either instance, heat generated by such combustion can be sensed and measured.
One known form of pellistor includes a coiled wire heater. Such sensors can be manufactured in relatively small sizes and exhibit relatively low power consumption. However, the cost and labor involved is excessive and increases with decreasing size. Another form of pellistor has been disclosed in U.S. Pat. No. 6,395,230 B1 entitled “Pellistor”, issued May 28, 2002. The '230 patent has been assigned to the assignee hereof and is incorporated herein by reference.
A number of methods have been employed to attempt to reduce the power consumption and/or manufacturing cost of pellistors. One example is the use of micromachined silicon ‘hotplates’ which can be made with small heated areas and hence lower power consumption. These devices have a number of disadvantages. The cost of development/redesign of micromachined silicon devices is high. The long-term stability of the heated substrate is poor at the elevated operating temperatures required (e.g., 550 degrees Celsius) and especially at the much higher temperatures required during manufacture of pellistors. The substrates are fragile, making deposition of the sensing material difficult. There are materials incompatibility issues—for example the ceramic materials normally used in pellistors (alumina/zirconia) have very different thermal expansion coefficients to substrate materials such as silicon or silicon nitride, which may result in poor adhesion and/or cracking under thermal cycling.
An alternative to silicon for microhotplates is silicon carbide, which has much better thermal stability than silicon, but can be difficult to etch. Another major disadvantage of silicon carbide is its high thermal conductivity, resulting in increased heat loss down the connecting struts of the hotplate compared with silicon/silicon nitride, giving higher power consumption for the same type of structure.
A substrate material that is commonly used for heated metal oxide gas sensors is alumina (Al2O3). Alumina has the advantages of being a much more refractory material than silicon, and is low cost and readily available. Alumina is also a commonly used substrate for screen printing, and commercial off-the-shelf screen printable metallisations and other materials compatible with alumina are readily available. Unlike silicon and silicon carbide, Alumina is difficult to micromachine on the small scales required to produce microhotplates, as it needs to be machined mechanically rather than photolithographically. Alumina is therefore typically used in the form of chips of dimensions of a few millimeters, and is typically attached to a suitable header using metal wires or lead-frames.
Alumina has a high thermal conductivity—this gives rise to a fairly uniform temperature across the device, regardless of the heater size. As a result the power consumption of such devices is excessive, and is unacceptably high compared to conventional wire-wound pellistor devices. An additional disadvantageous consequence is that the ends of the suspending wires or lead-frame connected to the sensing device are at high temperature which restricts the range of materials from which said wires or lead frame can be fabricated, and also can result in acceleration of thermally related failure modes in the wires or contacts between the wires and the substrate. In addition to having a high thermal conductivity, alumina substrates typically need to be relatively thick, for example 250 micrometers or more, in order to be sufficiently robust. This results in further lateral heat loss and hence higher power consumption. Methods have been developed to allow thermally insulated heated structures to be fabricated on alumina substrates. For example a thermally insulating glass layer can be printed between the alumina substrate and the heater+sensing layer. This approach has disadvantages. Since the glass layer is relatively thin and covers a relatively large area, heat loss is still significant. The glass layer may be less thermally stable than the underlying alumina substrate and may have a different coefficient of thermal expansion. Adhesion of the heater/electrodes/sensing material to the glass layer may be worse than to alumina. Patterning methods such as photolithography or laser trimming can be complicated by the presence of the additional layer.
It is desirable to further reduce power consumption for such gas sensors. Preferably the expenses of known coiled wire heated-type pellistors can be avoided. It is also desirable to minimize sensor poisoning.
As noted above, existing commercial catalytic bead flammable gas sensors (pellistors) are often made by winding a coil of fine platinum wire and depositing over this a relatively thick (hundreds of micrometers to several millimeters) porous catalyst/ceramic material. Larger devices generally have better poison resistance than smaller ones, this may be due to poisoning progressively destroying the activity of the sensor from the outside inwards. It is also possible that poisoning occurs at a uniform rate within the film but the target gas has a concentration profile, the steepness of which will decrease as the catalytic activity of the sensor decreases due to poisoning. In this case, a larger bead still gives improved poison resistance since the target gas is able to penetrate further into the sensor before being combusted. Whatever the mechanism, it is known that larger diameter beads typically have better poison resistance, albeit at the expense of greater power consumption.